Improving eyesight is vitally important. Precise measurement of the eye's physical characteristics, such as ocular refractive power, figures of surfaces including features of the eye in order to prescribe vision correction is also vitally important.
Since the Chou Dynasty (circa 479-381 B.C.) man has tried to correct his vision, knowing that the measurement of how much correction is required is a major part of the problem. Typically, in contemporary practice Snellen's charts are used with a phoropter to pragmatically and subjectively quantify the vision correction. This process relies on patient response to quantify the measurement. Auto refractors have been invented that use the knife edge test, myers and other optical principles, to quantify the visual acuity via light reflected from, or imaged on the retina. Optical characteristics of the eye are qualified by specific aberrations.
Currently, patient refraction measurements require verbal feedback from the patient in order to quantify the refraction measurement. Thus, in order to perform the measurement on both eyes simultaneously, the number of independent variables in the concurrent indicators allow too many degrees of freedom and thus there would be no accuracy in the refraction of either eye. Consequently, only one eye can be measured at a time. One of the enabling technologies of this invention is the ability to measure the refractive states, and thus the corrections required in a binocular mode of operation, i.e., both eyes simultaneously.
The technology of this invention is the result of treating the eye as an optical system. The optical train from the vertex of the cornea to the focal plane of the retina can be analyzed with the same fundamental techniques as sophisticated optical systems, that is, by analyzing the optical wavefront that passes through the system. In this invention a method for analyzing such an optical wavefront is disclosed.
A characteristic of the eye is needed in order to track its motion and strabismometry. Methods have been used that scar the cornea and track the scar. Tracking the inside edge of the iris is another technique that has been used, however the iris diameter changes with ambient light and ocular field of regard. Thus, the error induced as the result of iris tracking is larger than the magnitude of the motion measured, leaving it an invalid technique. This invention enables eye tracking by using geometrical characteristics of the entire pupil and a corneal glint to track the eye.
Classical techniques for measuring the quality of optics or optical designs are not suitable for measuring the optical performance of the eyes. A relationship between an impinging beam and the reflecting beam gives information about the eye's characteristics.
A method other than applying interferometry to measure optical characteristics is obtained from allowing light exiting the system to cast the shadow of a reticle. If the physical characteristics of the shadow pattern change are different than expected, the deviations can be analyzed and the anomalies can be quantified. This invention is particularly related to the analysis of light as it exits the eye post reflex from the ocular fovea. This reflex light must be appropriately conditioned, due to the reflective characteristics of the retina, so that it may be spatially coherent enough to provide the shadow conditions.
The method of this invention by which the optical wavefront at the vertex or the cornea is produced and analyzed is a primary feature of this invention. No means exists for accurately determining the optical characteristics of the eye in a continuous, real time and binocular (if desired) fashion. Moreover, Applicant is unaware of any techniques whereby reflections can be obtained from the retina and made to be cooperative, such that more specific measurements such as characteristics of features of the eye (i.e., refractive optical power state, corneal topography, corneal pachymetry, retinal acuity, ocular acuity, pupillometry, etc.) can be determined. More specifically, Applicant is unaware of any known ability to measure and analyze the data from a wavefront sensor. Such usefulness would include automatic evaluation of corrective lenses required for vision, characterization of eye motion and screening for acuity.
Processing the optical wavefront to provide measurement of the optical characteristics of the eye is valuable. This invention also relates to the processing and analysis of optical wavefront data. More specifically, the invention is directed to the optical wavefront containing information concerning characteristics related to the wavefront reflex from the retinal surface or corneal surface. Both optical and software means are used in individual and integrated forms to analyze the optical wavefront. Shadow patterns are produced when the optical wavefront is obscured in its propagation path by the pattern of a reticle such as a Rouchi ruling. These shadow patterns can also be made to produce lower spatial patterns by projecting them onto a second reticle. The resulting pattern is an interaction of two patterns that are near the same frequency, which is the superposition, interaction or interference of two simple harmonic functions that have different frequencies, be they electromagnetic, acoustic or spatial. In classical optics these patterns are know as Talbot interferograms, Fresnel patterns or moires. With such information, valuable refractive characteristics of the eye can be measured.
Fringe patterns are caused by a wave optics interferometric process. In one form of such process, a collimated light beam is divided so that part of the beam is directed towards a reference and another towards a target. Reflections from the reference and target interfere to provide an interferometric pattern. Interpretation of the pattern can provide measurement characteristics about the target. This technique is not applicable to the retinal reflex wavefront from live eyes since the geometrical relationship between the two optical paths must be held constant and spatial coherence must be maintained. This is not possible with a retina that is usually moving.
Another form of interferometric pattern is generated, in this invention, by the interference formed when two transparencies (e.g., grating-like), each with similar or identical regular patterns, overlap. The transmission of light through each transparency creates shadows. The fringe pattern is the shadow that is generated through the superposition of the shadows of the two separated transparencys' shadows. The interpretation of this pattern can provide useful information about geometric characteristics of an optical wavefront as reflected from different ocular interfaces. This is related to the reflection from the retina, the cornea and the endothelium.
The fringe pattern can be distorted by noise in the system generating the pattern. The noise can be electronically generated, caused by the camera and optical system used in the measurement, or by background or spurious light interference. Additionally, different reflectivity characteristics of the surface disassociated with the measurement being sought, can also impact accurate measurement. The reflectivity problems could arise, for instance, where different contrast characteristics of the surface exist. Alternatively, this can be caused by different background sources directed on the surface in a manner unrelated to light associated with the optical measuring system. A method for eliminating noise is provided in this system.